1. Field of the Invention
The present invention relates to a motor control circuit and, more particularly, to a motor control circuit having a motor protective circuit of a simple configuration, for detecting an overcurrent which flows to a motor due to a failure in a load and stopping the driving of the motor to thereby prevent the motor from being damaged.
2. Description of the Related Art
Conventionally, in a power window apparatus of a vehicle, a window opening/closing motor for opening/closing a window is used. When a window closing switch (window up switch) is operated, only during the operation, the window opening/closing motor rotates in one direction to move a window in the closing direction via a window operating mechanism coupled to the window opening/closing motor. On the other hand, when a window opening switch (window down switch) is operated, only during the operation, the window opening/closing motor rotates in the other direction to move the window in the opening direction via the window operating mechanism. In the case of operating both the window closing switch and an automatic window closing switch (automatic window up switch), even when the operation of the window closing switch and the automatic window closing switch is stopped, the window opening/closing motor continuously rotates in one direction to move the window to the full close position. Similarly, in the case of operating both the window opening switch and an automatic window opening switch (automatic window down switch), even when the operation of the window opening switch and the automatic window opening switch is stopped, the window opening/closing motor continuously rotates in the other direction to move the window to the full open position.
In the power window apparatus having such a configuration, when the load of the window opening/closing motor becomes extremely heavy due to a failure in a window operating mechanism or window itself and an overcurrent is passed to the window opening/closing motor, there is a fear such that the window opening/closing motor is burnt and smoking or firing occurs. In the motor control circuit for the window opening/closing motor, a motor protective circuit is assembled. When an overcurrent is passed to the window opening/closing motor, the motor protective circuit is operated to stop the driving of the window opening/closing motor.
In an existing motor control circuit for use in a power window apparatus, as a motor protective circuit, a current detecting resistor, a custom integrated circuit (ASIC) for motor control, and a comparator which are connected in series to the window opening/closing motor are used. The motor protective circuit takes the drive current of the window opening/closing motor passed to the current detecting resistor from both ends of the current detecting resistor as a voltage and comparing the taken voltage with a reference voltage by the comparator. When a comparison voltage outputted from the comparator becomes equal to or larger than a predetermined value, the driving current of the window opening/closing motor is controlled by the comparison voltage so as not to be equal to or larger than a predetermined value, thereby preventing the window opening/closing motor from being burnt.
The existing motor control circuit for use in the power window apparatus has the motor protective circuit for controlling the driving current of the window opening/closing motor so as not to be equal to or larger than a predetermined value, thereby preventing the window opening/closing motor from being burnt. As the components of the motor protective circuit, however, relatively expensive parts such as the custom integrated circuit (ASIC) for motor control and the comparator are used, so that the manufacturing cost of the motor protective circuit is high. Since the number of parts constructing the motor protective circuit is relatively large, at the time of manufacturing the motor control circuit, the number of assembling steps is large and the assembling procedure is complicated.
The invention has been achieved in consideration of such a technical background and its object is to provide a motor control circuit having a motor protective circuit of a simple configuration constructed by using the small number of cheap parts.
In order to achieve the object, according to the invention, there is provided a motor control circuit comprising: a motor coupled to a load; a current detecting resistor connected to the motor in series; a series circuit of an operation switch, a switching device, and a relay winding connected between a power source and a reference potential point; a bias circuit including a temperature dependent resistor for supplying a bias voltage to the switching device; a switching state self-holding circuit including an automatic operation switch; and an overvoltage detecting circuit, wherein when the operation switch is switched on or when the operation switch and the automatic operation switch are simultaneously switched on, the switching device is switched on to energize the relay winding, a contact of the relay is switched, and the motor is driven, and when an overcurrent is passed to the current detecting resistor and a temperature rise in the current detecting resistor is detected by the temperature dependent resistor and/or when a rise in a voltage across terminals of the current detecting resistor is detected by the overvoltage detecting circuit at the time of driving the motor, the switching device is switched off via the bias circuit to thereby stop the driving of the motor.
With such a configuration, the motor protective circuit for immediately stopping the driving of the motor when an overcurrent is passed to the motor in both the manual operation performed by the operation switch and the automatic operation performed by both the operation switch and the automatic operation switch is constructed by the current detecting resistor, temperature dependent resistor, switching device, and relay. Consequently, the number of component parts is smaller as compared with that of an existing motor protective circuit of this type and cheap parts can be used as the component parts. As a result, the configuration is simpler and the manufacturing cost is lower as compared with the existing motor control circuit.
In the motor control circuit having the above configuration, first and second sets each comprising: a series circuit of an operation switch, a switching device, and a relay winding; a bias circuit including the temperature dependent resistor; a switching state self-holding circuit including an automatic operation switch; and an overvoltage detecting circuit may be connected in parallel to the motor and the current detecting resistor. In the first or second set, when the operation switch is turned on or the operation switch and the automatic operation switch are simultaneously turned on, the switching device is switched on to energize the relay winding, and the contact of the relay is switched to thereby drive the motor. When an overcurrent is passed to the current detecting resistor and a temperature rise in the current detecting resistor is detected by the temperature dependent resistor in the first or second set and/or when a rise in a voltage across terminals of the current detecting resistor is detected by the overvoltage detecting circuit in the same set at the time of driving the motor, the switching device in the same set is switched off via the bias circuit in the same set to thereby stop the driving of the motor.
With such a configuration, at both of the time of displacing a member to be controlled in one direction and the time of displacing the member to be controlled in the other direction, the motor protective circuit for immediately stopping the driving of the motor when an overcurrent is passed to the motor in the two operations of the manual operation performed by the operation switch and the automatic operation by the simultaneously operation of the operation switch and the automatic operation switch can be constructed by the small number of cheap component parts.
In this case, in the invention, preferably, the switching device is a transistor, a bias resistor is connected between a base and an emitter or between the base and a collector, and when the operation switch is switched on, a self bias is applied to the base via the bias resistor to turn on the transistor.
With such a configuration, a general, cheap transistor is used as the switching device. Since the transistor is turned on by turning on the operation switch to apply the self bias to the base of the transistor via the bias resistor, the configuration of the motor protective circuit is simple and the cost of the motor protective circuit is low.
Preferably, the temperature dependent resistor included in the bias supplying circuit in the invention is either a positive temperature resistive element or a negative temperature resistive element.
With such a configuration, when a change in temperature of the current detecting resistor which occurs in association with a change in the motor driving current flowing through the current detecting resistor is detected by the temperature dependent resistor, the resistance value largely changes. A change in voltage of the bias supplying circuit becomes large, and the switching device can be controlled to be on or off with high sensitivity.
Further, in the invention, a window opening/closing motor for a power window apparatus can be selected as the motor, and a window opening switch or window closing switch for the power window apparatus can be selected as the operation switch.
With such a configuration, the small, cheap motor control circuit of a power window apparatus can be achieved. Moreover, by providing the motor protective circuit, the motor can be prevented from being burnt.